Personally I would first try to determine how much flow you have now with the gap and then after your mod whatever you might end up with. Putting a 1" non-tapered lip on the edge might do more harm to the venturi effect of the shroud than help? The corners of the shroud are somewhat not tapered already. Was just thinking of how any lip that was not tapered on say a carburetor venturi would hurt flow so would that also hurt on a fan shroud?? You could tape short pieces of (kite) string around the top part of the shroud before and after the mod like sometimes done in wind tunnels to check flow? Just maybe the morning coffee talking
Looks good btw
And is that F8 Green
Could always buy a larger diameter fan and cut it to size if the next size up is too large?
Mike